That's a really difficult number to estimate. I have payed 10-30 euro customs on some items (non hak5, just btw) that I re-ordered from the states. The same item, the same price, but totally different customs / fees afterwards.
I believe it depends on what the customs agents value the device at. One might value it more, one less. This should only happen if they don't believe what it says on the bill though..Otherwise depends on your carrier. Here in Germany it's 21% + handling fee for items over a certain value.